Project Summary/Abstract Synovial joints are essential for full range of motion and quality of life. Each joint is uniquely shaped to fit specific anatomical sites and permit relevant body movement and function. Unfortunately, the joints -and articular cartilage in particular- are highly susceptible to congenital-, injury- and age-related diseases that affect their structure and function, also a reflection of poor intrinsic joint repair capacity. In addition, current clinical interventions do not meet the wide range of demands on articular cartilage in vivo due, in large part, to lack of crucial knowledge on the mechanisms that govern normal growth and structure of the tissue. In order to advance these strategies, more information is needed on basic mechanisms of articular cartilage development, growth and morphogenesis. Were such information available, it would inform the improvement of clinical intervention, and bioengineering and gene therapy strategies as has been accomplished in disease- and injury- related treatment of many other human tissues. Articular cartilage is uniquely organized and displays distinct zones each providing specific and important functions for the synovial joint: (i) surface zone cells secrete lubricants into the joint cavity; (ii) deep zone cells are organized in columns to resist compressive loads; and (iii) a calcified cartilage zone provides attachment of the tissue to the underlying bone. Little is known about the mechanisms that drive this specific organization. Recent work from my sponsor and colleagues has improved our understanding of postnatal articular development using genetic lineage-tracing tools in mouse models. The data show that cell proliferation plays a more minor role in articular cartilage postnatal growth than previously thought and suggest, instead, that cell translocation and realignment are major drivers of growth and organization. Based on the results from these novel studies, I hypothesize that articular cartilage formation and growth mechanisms are primarily driven by cell intercalation and realignment into stacks and columns, permitting tissue thickening and organization. To test this hypothesis, in Aim 1 I will examine the spatio-temporal expression patterns of genes relevant to, and regulating, cell intercalation and alignment mechanisms. In Aim 2 I will test the function of core components of these pathways that have proven critical for organization and growth in other tissue contexts. I will use multiple analytical tools including histomorphometry, in situ hybridization, confocal imaging and cell culture in combination with mouse transgenic approaches. The proposed studies will provide essential knowledge on mechanisms that underlie the acquisition of articular cartilage structure and function. The data and insights from the project will prove essential to envision and test future therapeutic joint strategies, providing broad relevance and importance to the project and offering a powerful and encompassing platform for establishing my independent career in biomedical and translational medicine research.